Referring now to FIG. 1, an exemplary method for controlling torque of an engine is shown generally at 10. Torque control begins with step 12. In step 14, control determines if the engine is operating. If the engine is operating, instantaneous RPM (R), manifold absolute pressure (MAP), mass air flow (MAF) and air/fuel ratio (AF) are measured in step 16. If the engine is not operating, control ends in step 28. In step 18, a desired air per cylinder (APCdes) is estimated based on an inverse function of torque. APCdes is determined based on parameters measured in step 16 and other measured values. APCdes may be characterized by the following equation:APCdes=Tapc−1(Tref,R,S,D,AF,OT,Ω)  (1) Where Tref is reference torque, S is spark, D is dilution based on exhaust gas, OT is oil temperature and # is the number of cylinders of the engine. In step 20, engine torque is estimated by the following equations:T=ηof*ηΩ*(Tw+Tot)  (2) Tw=APCdes+aR*R+aS*S+aS*S2  (3) Where ηaf is the efficiency of air flow through the engine manifold,η# is the efficiency of the cylinders of the engine, Tw is the warm up torque of the engine, Tot is the initial torque of the engine and ai are coefficients.
In step 22, the desired MAF is calculated based on the follow:MAFdes=APCdes*R  (4) In step 24 the desired area is calculated based on the following:                               A          des                =                                            APC              des                        *            R            *                                                            R                  gas                                *                T                                                          15            *            B            *            Φ            ⁢                                                   ⁢                          (                              P                B                            )                                                          (        5        )            where R is the measured RPM, Rgas is the ideal gas constant, B is barometric pressure and P is the measured MAP. Equation (5) is hereinafter referred to as the compressible flow equation. Control then loops back to step 14.
As shown, desired area (Ades) is a function of RPM (R) and manifold pressure (P). Under transient conditions, the controller does not have lead information enabling fast torque control response. In this regard, an undesirable time delay may occur while correcting MAP and RPM to a desired level. For example, as illustrated in FIG. 1A, a time delay Δt may occur. In this way, MAP will grow from idle to wide open throttle causing a delay in area opening. As a result, equation (5) will not provide an instantaneous change in area. A similar undesirable time delay may also result during engine torque control for RPM correction.